So they attached small cameras—called Crittercams—to 42 monk seals to see if they could document the seals' interactions with fish.

Crittercam is a National Geographic Society-patented research tool for collecting video and audio recording and environmental data. (National Geographic News is part of the National Geographic Society.)

The Crittercams caught 69 hours of footage, including scenes of jack fish and sharks stealing prey—such as eels and octopi—that were uncovered by the seals.

"We saw seals swimming along, flipping rocks, and then jacks would swim up ahead and wait at the next rock," said NOAA biologist Frank Parrish.

Only about 1,200 Hawaiian monk seals still live in the wild, and the mammals are declining at a rate of 4 percent a year.

Scientists have been wondering why the protected waters northwest of Hawaii have been more hostile to the seals than the heavily fished seas around the main islands, where a much smaller monk seal population has grown slightly in recent years.